how to merge faces to make the final face subdividable
1/ is the face you are starting from
2/ is the result you have with a subdivision
3/ shows 1 triangulated (the inner geometry of the 1 ngon)
4/ the result that is obtained applying a subdivision to 3




You may proceed with a shape composed of 3 squares or rectangles, like this below :


1/ no more ngons but 3 quads
2/ the result after a subdivision